Continuous casting of rimming steel

ABSTRACT

In the production of steel by the continuous casting technique, a steel having a negatively segregated outer surface layer is made. An electromagnetic field is applied to the molten steel in the mould by coils surrounding the mould so that the steel is stirred with a rotary motion and passes over the outer solidifying layer in a direction parallel to the longitudinal axis of the cast strand and returns through the centre of the mould. The steel supplied to the continuous casting mould is in a deoxidised or partially deoxidised state.

United States Patent [191 Middleton et al. 5

[111 3,811,490 [451 May21, 1974 CONTINUOUS CASTING 0F RIMMING STEEL [75] Inventors: Malcolm Cecil Middleton, Dodworth nr. Barnsley; l-lenry Bassell Lloyd, Horsham, both of England [73] Assignee: British Steel Corporation, London,

England 221 Filed: Apr. 5, 1973 [2!] Appl. No.: 347,994

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 124,825, March 16,

197i, abandoned.

[52] U.S. Cl 164/49, 164/66, 164/82 [51] Int. Cl B22d 11/02, B22d 27/02 [58] Field of Search 164/49, 66, 82, 147, 259

[56] References Cited UNlTED STATES PATENTS 3,402,757 9/l968 Halliday 164/66 l2/l970 Keene 164/259 FOREIGN PATENTS OR APPLICATIONS l,l40,200 2/1957 France 164/49 1,807,435 l0/l970 Germany l64/49 Primary ExaminerRobert D. Baldwin Attorney, Agent, or FirmBacon & Thomas ABSTRACT 4 Claims, 8 Drawing Figures PATENTEDMAY 21 974 SHEET 1 OF 4 I- KEV/2 m/ve/vrozs M/ILCflLM C. MIDDLETON HENRY b. Lwru r4 TTORNEYS P N E-nmm I974 3.811.490

' sum 2 OF 4 FIG. 3.

/N VE N TOPS MALCOLM C. M/DOLE TON 1 HEN/2y 6. LLOYD PATENTEBMAYEI I974 3.81 90 SHEET 4 BF 4 F/G.6. FIG] f R R 3x9 7 e y/ s Y B Y /N VE N TOPS M/ILCOLM C. M/ODLETON HENRY 5 Mom ,9 TTORNEYS 1 CONTINUOUS CASTING OF RIMMING STEEL A continuous casting a steel having a negatively segregated outer surface layer.

When steel is produced by the conventional ingot casting method using undeoxidised molten steel of carbon content less than 0.25 percent the product is called rimming steel which has two basic characteristics:

1. There is a well-defined negatively segregated outer producing rimming steel, the molten steel in the present process may be a killed or deoxidised steel.

The molten steel may be stirred electromagnetically and is preferably stirred with a rotary motion about an axis extending transverse to the direction of movement of the steel through the mould. The molten steel may surface layer or rim around the ingot. By negatively segregated outer surface layer we mean that the surface layer has a lower percentage content of such elements as carbon, sulphur, phosphorus and oxygen as compared with the percentage content of the central portion of the ingot. The surface layer is also generally cleaner, in respect of the non-metallic inclusions such as oxides, than the central portion of the ingot. The characteristics of the surface layer facilitate subsequent treatment of steel, such as deep drawing or surface treatment.

2. The ingot contains blowholes both beneath the surface layer and in the centre of the ingot, which arise as a result of carbon monoxide evolution during solidification of steel. Thus pipe is not formed in the ingot, and a high percentage of the steel after primary rolling is suitable for further processing.

We have hitherto not been able to produce a negatively segregated outer surface layer when pouring undeoxidised steel into a continuous casting mould. Reference is made to R. Johnson et al., JISI, Special Report No. 89, (1964) entitled Continuous Casting at Appleby-Frodingham.

If steel in a generally undeoxidised condition is poured into a continuous casting mould, a sufficiently thick nonporous outer surface layer may be produced suitable for subsequent processing. However, large bubbles of carbon monoxide form in the liquid core of the metal and rise to'the upper surface causing surges in the surface level of the metal in the mould. This problem is most acute when casting slab thicknesses up to 200 millimetres.

If the steel is poured into the mould in a partially deoxidised condition, it is possible to reduce the problems associated with level control. However blowholes are formed so near the surface of the cast strand that they become exposed in subsequent processing and lead to surface defects in the steel product.

It should be noted that the presence of blow-holes in the steel may necessitate slow casting speeds since the blow-holes reduce the thermal conductivity of the steel thus reducing the solidification rate.

According to one aspect of the present invention a method for the production of steel in which the steel is poured into a continuous casting mould is characterised in that the molten metal in the mould is stirred in such a way that a negatively segregated outer surface layer is produced in the cast strand of steel encasing a core containing a majority of the impurities, i.e., a strand of steel is produced having characteristics approximating those of a rimming steel.

The steel used in this process is a low carbon molten steel of carbon content generally less than 0.25 percent. Unlike the conventional ingot casting method for also be stirred with a rotary motion such that it passes over the outer solidifying layer in a direction substantially parallel to the longitudinal axis of the cast strand and returns through the centre of the mould.

The steel poured into the mould may be in at least a partially deoxidised condition, or in a generally fully deoxidised condition. The pouring may be carried out in the presence of an inert gas to prevent reoxidation. Alternatively the pouring may be carried out in the presence of a gas of controlled oxygen potential to control reoxidation.

The stirring means is preferably adapted to apply electromagnetic forces to the molten metal and preferably is such that the molten metal is stirred with a rotary motion about an axis transverse to the direction of movement of the steel through the mould.

In the accompanying drawings:

FIG. 1 shows a schematic cross-section of one embodiment of apparatus for carrying out the method according to the invention.

FIGS. 2 and 3 show schematic cross-sections of further embodiments of apparatus for carrying out the method according to the invention.

FIGS. 4 and 5 show graphs of solute concentrations across the solid/liquid metal interface of solidifying ingots of a fully deoxidised and a substantially undeoxidised steel respectively, without any superimposed stirring. I

FIGS. 6, 7 and 8 show schematic representations of the electrical supply and connections to the apparatus.

The apparatus shown in FIG. 1 comprises a watercooled, open-ended, continuous casting mould l situated beneath a tundish 2. The mould 1 has a stirring means consisting of a coils 3 which are connected to a source of alternating electric current.

The current to the coils 3 is supplied by a three-phase star connection as shown in FIG. 8. Coils R and R are in the same phase, coils B and B are in the same phase and coils Y and Y are in the same phase. From FIG. 6 it can be seen that each coil is connected with a different phase of the three-phase supply from that phase which is connected with an immediately adjacent coil.

Liquid steel 4 in a fully deoxidised condition is poured from the tundish 2 into the mould l. The surface of the steel is cooled by the mould l and cast strand 6 is withdrawn in a known manner from the lower end of the mould. As the steel passes through the mould l, the coils 3 are switched on to set up an electromagnetic field within the mould l so'that vigorous stirring of the liquid metal 4 occurs in the direction shown by the arrows in FIG. 1, i.e., the molten steel is stirred with a rotary motion about an axis transverse to thedirection of movement of the steel through the mould 1. In the embodiments shown, the molten steel is stirred so that steel at the centre of the mould moves in the general direction of withdrawal of the cast strand, whilst steel near the internal periphery of the mould moves in the opposite direction.

The thickness of the negatively segregated outer surface layer thus produced is a function of the time taken by any horizontal plane to pass beyond the influence of the stirring coil. Thus either of the variables, coil length or withdrawal rate, acting independently or in combination, may be used to effect control of the outer layer thickness. If the speed of the strand is approximately 0.6 metres/minute and the length of the zone of influence of the coil is 0.6 metres, a negatively segregated outer surface layer of about 2.5 cm. thickness is produced.

Partially deoxidised steel may be supplied to the apparatus of FIG. 1. In the absence of stirring, blowholes would be formed very close to the surface of the metal, which would give rise to surface defects. Stirring is therefore necessary to create a negatively segregated and non-pourous outer surface layer between the surface and the blowholes.

An alternative apparatus is shown in FIG. 2. In this case the liquid steel 4 enters the mould 1 by way of a submerged nozzle 7. This enables a molten slag cover 8 to be placed on the surface on the metal in the mould l, which reduces the tendency of the surface metal to freeze and also prevents reoxidation of the steel, particularly when it is supplied to the mould in a partially deoxidised condition.

The apparatus shown in FIG. 3 may be used for fully deoxidised or partially deoxidised steels to prevent reoxidation of the steel. Between the tundish 2 and the mould l is a flexible gas tight bellows 9 extending between thetundish outlet and a cover 10 for the mould l. The cover 10 is retained on the mould by nuts and bolts 11, there being an annular gas-tight seal 12 between the cover 10 and the mould. The cover 10 is provided with an inspection port 13. The mould l, the cover 10 and the bellows 9 form a gas-tight enclosure, and an inlet 14'and outlet 15 are provided for the ad mission and removal of an inert gas or a gas of controlled oxygen potential to and from the enclosure.

In operation of the apparatus shown in' FIG. 3, steel 4 in a partially deoxidised condition is poured from the tundish 2 into the mould l. Inert gas or a gas of controlled oxygen potential is drawn continuously into and out of the gas-tight enclosure. The formation of large gas bubbles which leave the steel in the mould can be reduced by the presence of a controlled atmosphere above the steel, but there is a tendency for blowholes to be formed near the surface as in a partially deoxidised steel. An adequate degree of turbulence may be maintained by subjecting the molten metal to electromagnetic stirring, so as to produce the desired nonporous outer surface layer.

The coils which are used for generation of the electromagnetic field may be positioned within the mould, surrounding the mould, or above or below the mould, providing that the stirringcreated by the electromagnetic field takes place substantially in the mould.

It is not necessary to have a three-phase electrical supply to the coils which are used for electromagnetic 4 stirring. Any multi-phase system could be used.

Trials have been made to determine the effectiveness of the invention. A nominally 300 mm square mould 1.22 metres high and made from 6.4mm thick type 3041.. stainless steel was used. The mould was cooled externally by water sprays. A three-phase coil 460mm in height was raised during the casting of the ingot at a speed of 0.6 metres per minute, i.e., the speed of rise of liquid steel in the mould, in order to effectively simu late a continuous casting mould in operation.

Samples of the resulting ingot where taken at various percentage heights from the base of the ingot at a number of distances in from the surface of the ingot, and these samples were subjected to metallurgical analysis. Examination of the resulting ingots gave chemical segregation and inclusion distribution patterns showing clearly the production of a rim of relatively pure material encasing a core which contained the majority of the impurities. Sulphur prints of sections of the cast ingots showed an outer surface layer of relatively pure material, approximately 25mm wide around the ingot. Analysis of this material showed that the steel in the outer surface layer or rim was satisfactorily negatively segregated with respect to carbon, sulphur and phosphorus as compared with the metal core. In addition the steel in the surface layer was markedly cleaner with respect to non-metallic inclusions in general, but especially those inclusions with a diameter'of greater than 10g,

compared with the core of the ingot and with similar EXAMPLES 'Ingot l steel'specification En3 Height Distance C S P Si Mn from ingot surface 95% 12mm. 0.25 0.033 0.036 0.22 i 1.06 50mm. 0.29 0.041 0.041 0.22 1.10

50% 12mm. 0.23 0.031 0.035 0.21 1.05 50mm. 0.30 0.040 0.044 0.21 1.12

Height Distance Area Area No. of No. of

From Oxides v Sulphides Oxides per Sulphides Ingot 100 sq mm per 100 sq Surface mm 10 mm 0.010 0.133 94 12 1776 41 20mm 0.033 0.163 490 35 3387 123 95% mm 0078 0.248 1210 153 3015 573 mm 0.053 0.198 1103 24 2631 472 10mm 0.012 0.122 118 0 1876 53 20mm 0.019 0.160 177 18 2720 106 50% 70 mm 0.048 0.173 478 112 3133 413 80 mm 0.036 0.180 519 53 3027 425 Ingot 2 0.l% Carbon Killed Steel Height Distance C S P Si Mn From ingot Surface mm 0.09 @0 0.028 0.065 0.32 0.005 mm 0.08 0.042 0.027 0.060 0.34 75% 30 mm 0.14 0.054 0.032 0.060 0.34 0.007

40 mm 0.12 0.058 0.032 0.070 0.34 50 mm 0.10v 0.060 0.033 0.070 0.33

10 mm 0.10 0.046 0.031 0.060 0.30 0.004 50% 20 mm 0.09 0.047 0.030 0.060 0.33

30 mm 0.14 0.049 0.031 0.060 0.33 0.006 40 mm 0.12 0.055 0.034 0.070 0.33

10 mm 0.09 0.045 0.028 0.060 0.30 0.005 20 mm 0.044 0.028 0.060 0.34 mm 0.14 0.047 0.029 0.060 0.34 0.008

40 mm 0.11 0.050 0.031 0.070 0.34 50 mm 0.10 0.060 0.031

The inclusion distribution was very similar to that of We have thus found that a negatively segregated ingot l, with the marked increase in oxides and sul- 20 outer surface layer can be produced by method other phides'of a size greater than 10p. between 30 and 60 mm in from the surface of the ingot. lt has been suggested that when a metal solidifies rapidly, as in the case of a steel ingot, the concentration across the solid/liquid metal interface of a relatively low concentration solute element such as carbon, sulphur and phosphorus, is as shown in FIGS. 4 and 5 for deoxidised steels (otherwise known as killed steels) and undeoxidised steels (otherwise known as rimming steels) respectively, where:

C Solute concentration in the solid metal. C Solute concentration in the liquid metal. C Solute concentration at the liquid/solid metal interface. C Thickness of interface And C5 K0 C0 Where K Distribution Coefficient. e.g.

0.29 for Carbon 0.05 for Sulphur 0.18 for Phosphorus The graphs of FIGS. 4 and 5 are on the same scale 7' and for steels of similar composition, and it will be seen that C for the rimming steel is lower than C for the killed steel. The negatively segregated rim, with C low as compared with C is shown in FIG. 5. This is typical for a conventionally cast rimming steel.

It is thought that a lower C value for rimming steel results from the lower C value. It is further thought that the lower C value in rimming steel is a result of the scrubbing or stirring action of the carbon monoxide bubbles at the solid/liquid interface and the metal circulation associated with this.

than gas evolution, in situations where gas evolution causes serious problems e.g. level control, and where small blowholes are not necessary to prevent the formation of pipe.

We claim:

1. A method for the production of steel having the characteristics of rimming steel comprising pouring molten steel in at least a partially deoxidised condition and having a carbon content below 0.25 percent into a continuous casting mould, electromagnetically stirring the molten steel in the mould with a rotary motion about an axis extending transverse to the direction of movement of the steel through the mould so that the molten steel passes over the outer solidifying layer in a direction substantially parallel to the longitudinal axis of the cast strand and returns through the centre of the mould and a negatively segregated relatively nonporous outer surface layer is produced in the cast strand of steel, and solidifying said cast strand with said negatively segregated relatively non-porous outer layer encasing a core containing a majority of the impurities.

gen potential to control reoxidation. 

1. A method for the production of steel having the characteristics of rimming steel comprising pouring molten steel in at least a partially deoxidised condition and having a carbon content below 0.25 percent into a continuous casting mould, electromagnetically stirring the molten steel in the mould with a rotary motion about an axis extending transverse to the direction of movement of the steel through the mould so that the molten steel passes over the outer solidifying layer in a direction substantially parallel to the longitudinal axis of the cast strand and returns through the centre of the mould and a negatively segregated relatively non-porous outer surface layer is produced in the cast strand of steel, and solidifying said cast strand with said negatively segregated relatively non-porous outer layer encasing a core containing a majority of the impurities.
 2. A method according to claim 1 in which the steel poured into the mould is in a generally fully deoxidised condition.
 3. A method according to claim 1 in which the pouring is carried out in the presence of an inert gas to prevent reoxidation.
 4. A method according to claim 1 in which pouring is carried out in the presence of a gas of controlled oxygen potential to control reoxidation. 